1. Field of the Invention
The present invention relates to conferencing in telephony and the like in general and in particular to methods of providing conferencing. More particularly still, it relates to methods of providing conferencing without dedicated hardware, but utilizing and controlling existing components of a subscriber's terminal. Therefore, neither dedicated additional conferencing hardware, nor an external network conference bridge, is needed.
2. Prior Art
The simplest approach to conferencing involves applying fixed gain to each participant's transmitted signal, with the sum of the scaled signals being provided to each participant's receive (listening) path. In such a scenario, the background noise of all participants is accumulated in the received signals. If there are many participants, the noise can be excessive and unpleasant. There is also strong risk that echo signals will be sent back to the talker, with gain added. The amount of gain that can be applied is also seriously limited by loop stability criteria. Remote CO parties in a conference will often be interconnected over twice the connection loss compared to a point to point call. Given this extra loss, the amount of fixed gain allowed by stability criteria when using a simple summer is very often insufficient to meet level requirements for good quality speech.
Advantageous prior art conference talker switching decisions are based upon the order in which active talkers participate, not upon their level, thus treating all talkers more fairly. First-come, first-serve operation occurs wherein the most recently active pair of transmitted signals have Automatic GAIN control (AGC) applied, and subsequently mixed for redistribution. The presently active pair only hear one another while others hear both active talkers. Subsequent talkers break into the conversation when either of the two most-recently active talkers cease activity. Therefore, the background noise from a maximum of only two locations is heard at any time.
Better methods discriminate echo from speech, allowing the application of large quantities of gain without stability penalties. The only stability criteria that must be met involve the two presently-active talkers. All other participants are free to receive full gain as required. This is the method used in the present invention and in the United States prior art U.S. Pat. No. 4,648,108.
U.S. Pat. No. 4,648,108 granted Mar. 3, 1987 to Ellis et al. and entitled “Conference Circuits and Methods of Operating Them” discloses a conference circuit having a plurality of ports for a corresponding plurality of conferees. Associated with the ports is a control circuit which determines whether a conferee is active, i.e. talking, or dormant, i.e. listening. The circuit applies gain to the “active” signals and attenuates the “dormant” signals. When a listener starts to talk, the circuit switches his port to the “active” mode. Difficulties arise in determining when a listener becomes active, due to noise and echo with the speech signal. They are mitigated by comparing the signal from the port with an echo signal estimate derived from the echo return loss for the transmission path associated with the port. The arrangement takes account of differing echo levels for different transmission paths.
In U.S. Pat. No. 4,648,108 a microprocessor is used in the conference bridge, but all other hardware is additional and dedicated.
U.S. Pat. No. 4,648,108 is incorporated herein by reference and is useful as a background to the present invention and defines many of the terms used herein.